Fluid heating apparatus



May 30, 1950 E. G. BAALEY Erm.

FLUID HEATING APPARATUS 2 Sheets-Sheet 1 Filed June 15, 1945 ATTORNEY May 30, 1950 E. G. BAILEY x-:TAL

FLUID HEATING APPARATUS 2 Sheets-Sheet 2 Filed June 13, 1945 INVENToRs f r 6 Bai/ey $7 ADa/,o /l/. Hardy/*ome ATTORNEY Patented May 30, 1950 FLUID HEATING APPARATUS Ervin G. Bailey, Easton, Pa., and Ralph M. Hardgrove, Westfield, N. J., assignors to The Babcock da Wilcox Company, Rocklcigh, N. J., a corporation of New Jersey Application June 13, 1945, Serial No. 599,180

11 Claims. (Cl. 263-19) The present invention relates to the construction and operation of fluid heaters of the type in which a fluent mass or column of solid heat transfer material is circulated downwardly through a heating chamber, in which it is heated by the passage of gaseous heating fluid in heat transfer relation therewith, and then through a connecting passage of reduced flow area to and through a subjacent cooling chamber, in which it is cooled by heat transfer to a second fluid to be heated. Fluid heating apparatus of this type is disclosed and claimed in our prior application, Serial No. 502,580, filed Sept. 16, 1943, now Patent No. 2,447,306 of August 17, 1948.

Fluid heaters of the type described usually employ ceramic refractory materials as the heat transfer material and are capable of being continuously operated at substantially higher temperatures than is permissible withY metallic heat exchangers. Ordinarily, the material selected for the heat transfer medium will have softening and fusion temperatures substantially in excess of the temperatures contemplated in the operation of any specific application of the fluid heating apparatus. However, under some circumstances, the economics involved in 'the selection of the heat transfer material may make it desirable to use a lower cost material having a lower temperature differential between its softening and fusing temperature and the maximum designed operating temperature of the apparatus. Under such conditions it is desirable, if not essential, to avoid localized overheating in any portion of the heat transfer mass within the apparatus so as to prevent possible fusion of that material and consequent interruptions in the continuity of heater operation. Localized overheating of the material may be caused by any one of a number of conditions or a combination of such conditions which are dependent upon the construction and operation of the fluid heating apparatus. Some of these adverse conditions may be caused by incomplete combustion of the fuel used in producing the heating fluid employed within the apparatus, an inadequate distribution of the heating fluid when contacting with the heat transfer material, or aleakage of a fluid which may be a combustion constituent Y gases, which is characterized by its ability to operate continuously at elevated temperatures with a high overall thermal. efficiency and a 10W maintenance cost. A further and more specific object is the provision of fluid heating apparatus of the type described capable of attaining rapid and complete combustion of a fuel used therein for generating gaseous products of combustion used as the heating fluid, before the heating fluid comes into heat transfer relation. with the fluent mass of refractory heat transfer material. A further object is the provision of means for distributing the heating fluid within the fluid heater whereby the temperatures of the fluid will be substantially uniform throughout the area of initial contact with the heat transfer material. An additional object is to provide a construction of fluid heating apparatus whereby the tendency for leakage of fluids between the upper chamber and the lower chamber is avoided. The heating fluid for the upper chamber I0 is usually formed by gaseous products of combustion which in the apparatus described are advantageously produced in a separate fluid fuel fired furnace II and delivered to the chamber I0 through a. connecting passageway I5. The heat transfer material is heated to a relatively high temperature during its passage through the upper chamber I0, which heat content is transmitted during its passage through the lower chamber I2 by contact with a fluid to be heated flowing therethrough. The cooled heat transfer material leaves the lower chamber I2 through a discharge pipe or spout I6 connected at its lower end to a suitable continuous feeder I'I. The feeder regulates the rate of withdrawal of material from the chamber I2 and discharges it into a continuous bucket elevator I8 which elevates and returns the heat transfer material through a spout 20 to the upper chamber I 0 for reuse in the heat exchange process.

A relatively wide range of refractory materials can be used as the fluent mass of solid heat transfer material II, the material selected depending upon the particular operating conditions to be maintained within the fluid heating unit. In general, the material should have a high strength and hardness, substantial resistance to thermal shock, and a high softening temperature. Such materials may be natural or manufactured ceramic refractories, corrosion resistant alloys and alloy steels, in small pieces of regular or irregular shape. As disclosed in said prior application, substantially spherical pellets of manufactured ceramic refractories have been successfully'used. The pellets should be of a size such as to provide a large amountof surface area for transfer of heat and of a density suicient to withstand the fluid flow velocities through the pelletl mass while in the heating and cooling chambers without lifting. One desirable size of ceramic refractory pellet has been found to be approximately 1; inches in diameter, but the size may vary above and below that value with the individual application of the fluid heater.

As shown in Fig. l, the upper chamber I is defined by a cylindrical metallic casing 2I having a domed top 22 and an inverted dome-like bottom 23, with all the interior surfaces thereof protected by a lining 24 of one or more layers of high temperature refractories. The lining of the domed top is formed by a monolithic poured refractory material and is provided with a light gauge metallic plate 25 suspended beneath the lower surface of the refractory lining so as to prevent the addition of any dislodged portions of the poured refractory lining to the bed of heat transfer material II within the chamber I0. In addition to the refractory lining, the cylindrical casing 2I is further protected by a layer of insulating material 26 which is interposed between the casing 2l and the refractory lining 24. An inlet 21 is provided in the center of the domed top 22 for the admission of heat transfer material and an outlet 28 adjacent thereto for the discharge of spent heating gases. The outlet 28 is connected with a stack (not shown) by means of a conduit 30 with the gas flow therethrough controlled by a valve 3|. The bottom of the chamber space for the mass of heat transfer material is defined by an inverted conical refractory structure 32 having its upper end circumferentially merging into the lining 24 of casing 2| while its tapered lower end terminates at the upper end of the throat I3. i

The inner surface of the conical bottom structure 32 is constructed at an angle substantially greater than the angle of repose of the solid material II, e. g. an angle of approximately 60 to the horizontal, and is formed of a plurality of tuyre blocks 33 shaped to form segments of a complete frusto-conical ring. Successive rings of tuyre blocks are placed between imperforate end rings 34 and 35`of refractory material. As shown in perspective in Fig. 4 and in section in Fig. 2, each tuyre block is provided with a symmetrically arrange group of four inwardly flared passageways 36 of rectangular crosssection therethrough. Obviously the blocks may be constructed of any convenient size and having one or more passageways therethrough. For example, the lower rows of passageways in the vicinity of the ring 34 may be formed in tuyre blocks of the general type illustrated while the upper rows of passageways in the vicinity of the ring 35 may be formed in tuyre blocks having only one passageway per block. The tuyre blocks are installed in adjoining rows with the larger end of the passageway facing inwardly and the upper flared side of the passageway substantially horizontal. With this arrangement of the tuyre blocks, the inner end of each passageway 36 will be lled with the heat transfer material which, following its normal angle of repose, will extend outwardly in the passageway to a position approaching the smaller rectangular opening in the outer or fluid inlet side of the blocks. Any tendency for the heat transfer material to flow outwardly through the smaller end of the passageway, as may be present with the use of a heat transfer materialhaving a small angle of repose, can be prevented by making the outer end of the passageway opening slightly less than the diameter of the individual pieces of heat transfer material.

A vertically arranged annular wall 31 of cir` cular or semi-circular high temperature refractory pieces 38 defines the upper portion of the throatl3 and extends downwardly from the ring 34 through the bottom of the upper chamber III to the lower chamber I2 as hereinafter described. A second vertical annular wall 40 of high temperature refractory material closely encircles the wall 31 and is extended upwardly from the horizontal lining 24' to a position approximately the level of the upper end of the wall 31. The wall 40 provides a support or annular pier for the lower end of an upwardly and outwardly flaring annular arch member 4I formed by rows of wedge shaped refractory blocks 42, which are provided with a plurality of ports 43 therethrough. Such ports may be rectangular in transverse cross-section, as shown in Fig. 3, or may be of any other shape having suilicient cross-sectional area for the ilow of heating gases.

A substantially stationary mass of spherically shaped refractory pieces 44 fill the space between the top of the arch member 4I and the conical structure 32. The diameter of the individual pieces 44 is preferably greater than that of the individual pieces of heat transfer material II so that they cannot pass through the tuyre block passages 33. With this construction, heating gases delivered to the annular space 45 between the wall 43 and the lining 24 will ilow through the openings 43, through the interstices of the pieces 44 and the passageways 36 into the mass of heat transfer material II normally occupying the chamber I0. The upper heating chamber is thus advantageously constructed to thoroughly diffuse heating gases received therein and effect a substantially uniform distribution of the heating gases into contact with the mass of heat transfer material II.

It is also desirable to arrange for the introduction of the heating gases into the annular space 45 so that the entering gases will have an equalized temperature. Where the products of combustion of a. burning fuel are used as the heating fluid in the apparatus, it is highly desirable to effect complete combustion of the fuel before introducing the resulting gases into the upper chamber III. This procedure will avoid any possibility of secondary combustion within the chamber I0 and consequently any localized overheating of the heating transfer material from this source. For this reason-the fuel is preferably burned in one or more separate furnaces I4 connected to the annular space 45 as shown in Fig. 1 and hereinafter described, and combustion air supplied to the furnace in an amount in excess of the theoretical combustion requirements.

The furnace I4 has a vertically elongated combustion space, circular in horizontal cross-section, and a side outlet 46 near the top thereof communicating with the annular space 45 through the horizontally" extending cylindrical refractory lined passageway I5. The longitudinal axis of the passageway l5 is shown as normal to and intersecting the vertical axes of both the furnace I4 and the upper pebble chamber Ill. The furnace is enclosed by a metal casing 43 and a nre brick lining 5I) which is separated from the casing 4I by a double layer of insulating lire brick 5I. The lower end of the lining is extended to form a horizontal refractory ring 52 which defines a burner port 53. A suitable fluid fuel burner 58 is axially mounted in the port 53. The metal casing 48 of the furnace I4 extends below the ring 52 and is shaped into a rounded end having a centrally located circular opening 54. A metallic cylinder 55 having a flanged lower end is inserted in the opening 54, the upper end of the cylinder 55 having a circular series of air deflecting vanes 56 forming an air register for the port 53. A circular plate 51 closes the lower end of the cylinder except for a central hub through which the burner fuel supply pipe extends. The air required for the combustion of the fuel delivered by the burner 58 is supplied by a forced draft fan 6I) through a valved duct 6| and intro'- duced to the furnace chamber through the air vanes 56.

The fuel and air discharged upwardly through the port 53 are ignited and burned in the furnace i4. It has been found that a ratio of length to diameter of the furnace of approximately 2% to 1 will not only permit complete combustion of the fuel before reaching the outlet 46, but will also avoid stratification of the gases produced. When those gases pass through a 90 turn, as in changing from a vertical to a horizontal direction of flow in the arrangement illustrated, the gases are further mixed, and enter the annular space 45 as a non-stratified high temperature heating gas stream having a substantially uniform temperature and composition throughout a cross-section normal to the direction of gas flow. As the gases pass through the openings, they divide into a multiplicity of smaller gas streams which are further diffused by passing through the interstices of the spherical pieces 44 and the passageways 36 in the tuyre blocks 33. lThe described tortuous paths taken by the heating gas in nowing from the annular space 45 into heat transfer contact with the heat transfer material I I within the upper chamber IIJ tend to diiluse and remix the heating gases so as to assure a desirable uniformity of temperature in those gases and thereby create a substantially uniform high temperature for the heat transfer material passing through the throat I3 into the lower chamber I2.

The connectingthroat passage I3 is defined by the refractory pieces 38 extending from the lower end of the tuyre block structure 32 to a level within the lining of the upper portion of the lower chamber I2. A tube 62 of Wear resistant highly refractory material, such as silicon carbide, is attached to the lower end of the pieces 38, thus providing an extension to the passage I3 which projects to a level below the upper end of the chamber I2. V The dimensions of the vertically elongated throat I3 are such as to provide a length sufficient when filled with the heat transfer material to substantially restrict gas flow between the chambers ID and I2, and yet provide a diameter sufficient to permit free flow of the heat transfer material I I from the upper to the lower chamber. Pressure tap connections 63 and 64 are provided at vertically spaced points in the throat I3, as disclosed in our said application, and the differential pressure changes therebetween are advantageously used to regulate the fluid pressure in chamber III to maintain a predetermined pressure differential, such as substantially balanced pressures, between the bottom of chamber I8 and the top of chamber I2.

The lower chamber I2 is circular in horizontal cross-,section and of substantially uniform diameter from its upper end to a downwardly tapered conical bottom. As shown in Fig. 1, the chamber I2 is encased in a metallic casing 66 which is lined by a protective layer of insulating material 61 and an inner wall 68 of high temperature fire brick. An inverted frusto-conical metallic plate 10 is attached to the lower end of the refractory lining of the chamber I2 to form a downwardly tapering lower portion of the chamber. The plate 10 is provided with a central outlet opening 1I for heat transfer material at its lower end and' the lower portion of the plate 10, which may represent approximately one third to one half of the total area thereof, is perforated to permit the flow of the iluidto be heated inwardly and upwardly therethrou-gh. The dimensions of the individual perforations are selected so as to prevent the passage of normal sized pieces of `heat transfer material outwardly through the holes.`

A corresponding portion of the casing 66 is also downwardly tapered to a central discharge opening 12 spaced below the corresponding outlet 1I of the plate 10. The spaced cones define an annular fluid inlet chamber 13 therebetween to which one or `more fluid supply pipes 14 having regulating valves 15 therein are connected for the admission of the fluid to be heated.

The upper end of the casing 66 is formed by a domed top section 16 lined with layers of insulating material and ilre brick. Ihe metallic casings 2I and 66 for the upper and lower chambers are spaced apart except for short center sections thereof surrounding the throat which are connected by a metallic expansion joint 11. With the throat tube 62 extending into the chamber I2, the heat transfer material delivered through the throat I3 will normally have an upper surface contour substantially as shown in Fig. l, and the space between the upper surface of the heat transfer material II and the lower surface of the top 16 will be free to receive the heated fluid flowing upwardly through the interstices of the heat transfer material II. Such heated fluids will pass through one or more refractory-lined fluid discharge pipes 18 opening to the chamber I2 through outlet ports 80 in the top 16.

The continuous feeder I1 receiving the pellets of heat transfer material from the discharge spout I6 is preferably of the variable speed rotary plate type disclosed in a copending application of A. M. Kohler, Serial No. 569,251, filed Dec. 2l, 1944, now Patent No. 2,468,712 of April 26, 1949, and may be used with or without sealing valves depending upon the character of the iluid being heated.

Since dust or fragments from the circulating heat transfer material and/or of the refractory lining of the apparatus may be created during the normal operation of the unit, it is usually desirable to clean or scavenge the heat transfer material as it re-enters the chamber I0. The

lower end of the spout 2U is arranged to receive part of the stack gases which pass through the descending heat transfer material entering the chamber I0. The resulting mixture of dust and gases flows to an external cyclone separator 8|. The separator has a bottom outlet for the separated solids and a vent pipe 82 connected to a l stack (not shown) for venting the gases. The

flow of scavenging gases is regulated by a valve 83 in the pipe 82.

The fluid heating apparatus illustrated is ordlnarily operated with an internal fluid pressure slightly above that of the surrounding atmosphere, but it may be operated with internal pres-l pension and contraction' of refractory linings occurring during the operation of the apparatus resulting from the elevated temperatures encountered, it has been found advantageous to enclose the chambers in gastight steel casings t avoid leakage of the contained fluids. Experience has shown that if the insulation and/or refractory materials surrounding the throat I3 also extend between the upper and lower chambers IB and I2 for the full transverse cross-section thereof, a limited flow of fluid through the wall structure will occur, with the amount of such flow depending upon the differential pressure between the chambers and the porosity of the intervening refractories. Such movement of fluid, or leakage, will usually be from the lower to the upper chamber and, may under some operations, adversely effect the operation of the unit, either through the loss of a valuable fluid or by hindering close pressure and temperature control of the fluids. Such adverse effects due to leakage through the wall structure from one chamber to the. other are largely avoided by reducing the outer diameter of the circular wall surrounding the throat section between the chambers I0 and I2, as shown in Fig. 1, and enclosing the parts in fluidtight metallic casings.

In operation the mass of heat transfer pellets fills the upper and lower chamber substantially to the levels indicated in Fig. 1. The pellets are moved through the chambers I0 and I2 and intervening throat I3 by operating the feeder Il at a substantially continuous predetermined rate which may be varied as desired to accommodate changed temperature requirements in the apparatus. Leaving the feeder, the pellets are elevated and returned to the upper chamber to repeat the cyclic heating and cooling thereof. The temperature of the heated fluid may be regulated by altering the rate of pellet flow through the heat ,exchange chambers of the apparatus and/or by suitable regulation of the fuel and combustion air quantities delivered to the furnace Il. The combustion air may be delivered to the furnaceat any desired temperature ranging from the ambient room temperatures to values in excess of 2000 F. Such highly preheated combustion air may be obtained in the manner disclosed in a copending application by C. L. Norton, Serial No. 543,442, flied July 4, 1944, or from any other available source.

It will be noted that in accordance with our invention the heating fluid utilized in fluid heaters of the type described is produced in an external furnace by the complete combustion of the fuel constituents and discharged to the upper chamber as a uniform temperature gas stream. The' described gas inlet section of the upper chamber insures a uniformly distributed delivery or the heating gases to the descending mass of heat transfer material, avoiding both localized under-heating and over-heating of the material.

In addition, the apparatus is so constructed that leakage of fluid through the connecting wall structure between the upper and lower chambers is reduced to a negligible amount during normal operation.

We claim:

1. A fluid heater comprising walls dening an upper chamber enclosing a fluent mass of refractory heat transfer material and having a. heating gas outlet at its upper end, a lower chamber enclosing a fluent mass of refractory heat transfer material and having a heat transfer material outlet at its lower end, and a structurally unobstructed throat passage of reduced cross-section connecting` said upper and lower chambers and enclosing a. fluent column of refractory heat transfer material connecting the masses of material in said upper and lower chambers, means for passing a fluid to be heated under a positive pressure through said lower chamber through the mass of heat transfer material therein, a separate vertically elongated funace chamber arranged to pass a heating fluid under a positive pressure through an outlet at one side of an endthereof and through the mass of heat transfer material in said upper chamber, the walls defining said upper and lower chambers having an outer metallic fluid-tight casing lined with refractory material, and a fluid-tight expansion joint surrounding said throat passage and connecting the metallic casings fory said upper land lower chambers.

2. A fluid heater comprising walls defining an upper chamber enclosing a fluent mass of refractory heat transfer material and having a heating gas outlet at its upper end, a lower chamber enclosing a fluent mass of refractory heat transfer material and having a. heat transfer material outlet at its lower end, a throat passage of reduced cross-section connecting said upper and lower chambers and enclosing a fluent column of re fractory heat transfer material connecting the masses of material in said. upper and lower chambers, an annular wall of refractory material encircling said throat passage between said upper and lower chambers and having an outside diameter substantially less than that of said chambers, means for passing a. fluid to be heated through said lower chamber through the mass of heat transfer material therein, a separate vertically elongated furnace chamber arranged to pass a heating fluid under a positive pressure through an outlet at one side of the upper end thereof and through the mass of heat transfer material in said upper chamber, the walls defining said upper and lower chambers having an outer metallic fluid-tight casing lined with refractory material, and a fluid-tight metallic expansion joint surrounding said throat -passage and encircling wall and connecting the metallic casings for said upper and lower chambers.

3. A heat exchange device comprising a chamber enclosing a fluent column of solid material and having a gaseous fluid outlet at its upper end, means for effecting movement of said solid material through said chamber, and means for passing a gaseous heating fluid upwardly through said 0 fluent mass of solid material within said chamber comprising tuyre lblocks arranged to form a perforate tapered bottom for said chamber, an inperforate annular wall defining a solid material outlet at the lower end of said chamber, a perforate annual wall surrounding the upper portion of said outlet and spaced from said tapered bottom, an interstitial mass of refractory material between said perforate wall and bottom, and means including said annular perforate wall defining an annular space surrounding said outlet and receiving a gaseous heating uid from an inlet at one side thereof.

4. A heat exchange device comprising a chamber enclosing a fluent mass of solid material and having a gaseous fluid outlet at its upper end,

9 means for effecting movement of said solid material through said chamber, and means for passing a gaseous heating fluid upwardly through said fluent mass of solid material within said chamber 1 5 cross-section at its lower end, and means for comprising tuyre blocks arranged to form a perforate inverted frusto-conical bottom for said chamber, a vertically inclined tube defining a solid material outlet at the lower end of said chamber, a Iperforate annular arch surrounding the upper portion of said tube and spaced from said inverted frusto-conical bottom, an interstitial mass of refractory material between said perforate arch and inverted frusta-conical bottom. and means including said annular perforate arch delining an annular space surrounding said tube and receiving a gaseous heating fluid from an inlet at one side thereof.

5. A solid and gas contact apparatus comprising walls defining a vertically elongated chamber of substantially uniform circular cross-section, a plurality of tuyre blocks forming a perforate tapered wall intermediate the longitudinal axis of said chamber ending in an imperforate annular wall defining an outlet of reduced circular crosssection from said chamber, means for maintaining a fluent mass of solid material within said chamber and supported on said perforate tapered wall, a perforate annular arch spaced below said tapered wall, an interstitial mass of solid bodies between said perforate tapered wall and perforate arch, and means for introducing gaseous fluid into said chamber through said perforate arch, interstitial mass and perforate wall.

6. A solid and gas contact device comprising walls dening a chamber enclosing a fluent co1- umn of solid material and having a gas outlet and a solid material inlet at the top thereof, a perforate inverted frusto-conical hopper bottom to said chamber supporting the fluent solid material therein and ending in a centrally positioned outlet at the bottom thereof, an imperforate substantially vertical tube in communication with said outlet and providing a passageway therethrough for said fluent column of solid material, a perforate annular arch spaced from said hopper bottom and extending from a position encircling the upper portion of said tube to an extension of the walls of said chamber, an interstitial mass of solid material between said perforate hopper bottom and said perforate arch, and an annular space defined bv said tube, said perforate arch, an extension of said chamber wall and a bottom closure encircling said tube arranged to receive a gas under a positive pressure and to discharge said gas through said perforate arch, interstitial mass and perforate hopper bottom into an intimate contact with the fluent column of fluent solid material within said chamber.

T. A fluid heater comprising a chamber enclosing a fluent mass of refractory heat transfer material and having a heating'gas outlet at its upper end and a discharge passage of reduced cross-section at its lower end, and means for heating the mass of heat transfer material in said chamber to a substantially uniform temperature throughout any cross-sectional area thereof comprising means forming a perforate tapered bottom for said chamber, a perforate wall spaced from said tapered bottom, a sas pervious interstitial mass of refactory material between said wall and bottom, a gas inlet space at the outer side of said wall, and means for supplying heating gases to said space at a predetermined and` substantially uniform temperature longitudinally and transversely of its direction of ow.

8. A fluid heater comprising a chamber enclosing a uent mass of refractory heat transfer material and having a heating gas outlet at its upper end and a discharge passage of reduced passing a. substantially uniformly distributed ow of heating fluid through the mass of heat transfer material in said chamber comprising a perforate tapered bottom for said chamber, a per- 10 forate annular arch surrounding the upper p0rtlon of said discharge passage and spaced from said tapered bottom, and having an annular space surrounding said annular arch, an interstitial mass of refractory material between said annular l5 arch and tapered bottom, and a separate furnace chamber having fuel burning means at one end thereof and a heating gas outlet at the other end thereof connected to said annular space.

9. A fluid heater comprising a chamber enclosing a fluent mass of refractory heat transfer material and having a heating gas outlet at its upper end and a heat transfer material outlet of reduced cross-section at its lower end, means for causing a movement of said fluent solid mate- 26 rial through said chamber, and means for passing a heating fluid through the mass of heat transfer material in said chamber comprising tuyre blocks forming a perforate tapered bottom for said upper chamber, a perforate annular 30 wall surrounding said heat transfer material outlet and spaced from said tapered bottom and having an annular space surrounding said annular wall, an interstitial mass of refractory material between said wall and bottom, and a separate vertically elongated furnace chamber having fue! burning means at the lower end thereof and a heating gas outlet at one side of the 'upper end thereof connected to said annular space.

10. A fluid heater according to claim 3. wherein 40 said tuyre blocks are each shaped as a segment of a frusto-conical ring and are arranged in a plurality of adjoining rings to form said chamber bottom, the top and bottom surfaces of the openings in said tuyre blocks diverging in the direction of gaseous heating fluid ilow therethrough with the top surface of each opening substantially horizontal.

11. A fluid heater according to claim '1 wherein the openings in said perforate tapered bottom are 5o rectangular in cross-section and increase in cross-sectional area inwardly of said. chamber, and each opening is defined by a substantially horizontal upper surface with a downwardly inclined lower surface and side wall surfaces converging inwardly of said chamber.

ERVIN G. BAILEY. RALPH M HARDGROVE.

REFERENCES CITED The following references are of record in the ille of this patent:

UNITED STATES PATENTS 

